321 research outputs found
Feasibility of Impact-Acoustic Emissions for Detection of Damaged Wheat Kernels
Cataloged from PDF version of article.A non-destructive, real time device was developed to detect insect damage, sprout damage, and
scab damage in kernels of wheat. Kernels are impacted onto a steel plate and the resulting acoustic
signal analyzed to detect damage. The acoustic signal was processed using four different methods:
modeling of the signal in the time-domain, computing time-domain signal variances and maximums
in short-time windows, analysis of the frequency spectrum magnitudes, and analysis of a derivative
spectrum. Features were used as inputs to a stepwise discriminant analysis routine, which selected a
small subset of features for accurate classification using a neural network. For a network presented
with only insect damaged kernels (IDK) with exit holes and undamaged kernels, 87% of the former
and 98% of the latter were correctly classified. It was also possible to distinguish undamaged, IDK,
sprout-damaged, and scab-damaged kernels.
© 2005 Elsevier Inc. All rights reserved
Effects of Velocity Correlation on Early Stage of Free Cooling Process of Inelastic Hard Sphere System
The free cooling process in the inelastic hard sphere system is studied by
analysing the data from large scale molecular dynamics simulations on a three
dimensional system. The initial energy decay, the velocity distribution
function, and the velocity correlation functions are calculated to be compared
with theoretical predictions. The energy decay rate in the homogeneous cooling
state is slightly but distinctively smaller than that expected from the
independent collision assumption. The form of the one particle velocity
distribution is found not to be stationary. These contradict to the predictions
of the kinetic theory based on the Enskog-Boltzmann equation and suggest that
the velocity correlation is already important in the early stage of homogeneous
cooling state. The energy decay rate is analysed in terms of the velocity
correlation.Comment: 9 pages (figures included). To be published in J. Phys. Soc. Jpn.
Vol. 73 No. 1 (2004) Added two references and removed one. Changed the name
of T_{L}. Added unit constants in Sec. 5 and
Hydrodynamic singularities and clustering in a freely cooling inelastic gas
We employ hydrodynamic equations to follow the clustering instability of a
freely cooling dilute gas of inelastically colliding spheres into a
well-developed nonlinear regime. We simplify the problem by dealing with a
one-dimensional coarse-grained flow. We observe that at a late stage of the
instability the shear stress becomes negligibly small, and the gas flows solely
by inertia. As a result the flow formally develops a finite time singularity,
as the velocity gradient and the gas density diverge at some location. We argue
that flow by inertia represents a generic intermediate asymptotic of unstable
free cooling of dilute inelastic gases.Comment: 4 pages, 4 figure
Hyperfine Populations Prior to Muon Capture
It is shown that the 1S level hyperfine populations prior to muon capture
will be statistical when either target or beam are unpolarised independent of
the atomic level at which the hyperfine interaction becomes appreciable. This
assertion holds in the absence of magnetic transitions during the cascade and
is true because of minimal polarisation after atomic capture and selective
feeding during the cascade.Comment: (revtex, 6 preprint pages, no figures
Close-packed floating clusters: granular hydrodynamics beyond the freezing point?
Monodisperse granular flows often develop regions with hexagonal close
packing of particles. We investigate this effect in a system of inelastic hard
spheres driven from below by a "thermal" plate. Molecular dynamics simulations
show, in a wide range of parameters, a close-packed cluster supported by a
low-density region. Surprisingly, the steady-state density profile, including
the close-packed cluster part, is well described by a variant of Navier-Stokes
granular hydrodynamics (NSGH). We suggest a simple explanation for the success
of NSGH beyond the freezing point.Comment: 4 pages, 5 figures. To appear in Phys. Rev. Let
Granular Rheology in Zero Gravity
We present an experimental investigation on the rheological behavior of model
granular media made of nearly elastic spherical particles. The experiments are
performed in a cylindrical Couette geometry and the experimental device is
placed inside an airplane undergoing parabolic flights to cancel the effect of
gravity. The corresponding curves, shear stress versus shear rate, are
presented and a comparison with existing theories is proposed. The quadratic
dependence on the shear rate is clearly shown and the behavior as a function of
the solid volume fraction of particles exhibits a power law function. It is
shown that theoretical predictions overestimate the experiments. We observe, at
intermediate volume fractions, the formation of rings of particles regularly
spaced along the height of the cell. The differences observed between
experimental results and theoretical predictions are discussed and related to
the structures formed in the granular medium submitted to the external shear.Comment: 10 pages, 6 figures to be published in Journal of Physics : Condensed
Matte
The Granular Phase Diagram
The kinetic energy distribution function satisfying the Boltzmann equation is
studied analytically and numerically for a system of inelastic hard spheres in
the case of binary collisions. Analytically, this function is shown to have a
similarity form in the simple cases of uniform or steady-state flows. This
determines the region of validity of hydrodynamic description. The latter is
used to construct the phase diagram of granular systems, and discriminate
between clustering instability and inelastic collapse. The molecular dynamics
results support analytical results, but also exhibit a novel fluctuational
breakdown of mean-field descriptions.Comment: 15 pages, 4 figure
Dynamics of Freely Cooling Granular Gases
We study dynamics of freely cooling granular gases in two-dimensions using
large-scale molecular dynamics simulations. We find that for dilute systems the
typical kinetic energy decays algebraically with time, E(t) ~ t^{-1}, in the
long time limit. Asymptotically, velocity statistics are characterized by a
universal Gaussian distribution, in contrast with the exponential high-energy
tails characterizing the early homogeneous regime. We show that in the late
clustering regime particles move coherently as typical local velocity
fluctuations, Delta v, are small compared with the typical velocity, Delta v/v
~ t^{-1/4}. Furthermore, locally averaged shear modes dominate over acoustic
modes. The small thermal velocity fluctuations suggest that the system can be
heuristically described by Burgers-like equations.Comment: 4 pages, 5 figure
The song of the dunes as a self-synchronized instrument
Since Marco Polo (1) it has been known that some sand dunes have the peculiar
ability of emitting a loud sound with a well defined frequency, sometimes for
several minutes. The origin of this sustained sound has remained mysterious,
partly because of its rarity in nature (2). It has been recognized that the
sound is not due to the air flow around the dunes but to the motion of an
avalanche (3), and not to an acoustic excitation of the grains but to their
relative motion (4-7). By comparing several singing dunes and two controlled
experiments, one in the laboratory and one in the field, we here demonstrate
that the frequency of the sound is the frequency of the relative motion of the
sand grains. The sound is produced because some moving grains synchronize their
motions. The existence of a velocity threshold in both experiments further
shows that this synchronization comes from an acoustic resonance within the
flowing layer: if the layer is large enough it creates a resonance cavity in
which grains self-synchronize.Comment: minor changes, essentially more references
On the velocity distributions of the one-dimensional inelastic gas
We consider the single-particle velocity distribution of a one-dimensional
fluid of inelastic particles. Both the freely evolving (cooling) system and the
non-equilibrium stationary state obtained in the presence of random forcing are
investigated, and special emphasis is paid to the small inelasticity limit. The
results are obtained from analytical arguments applied to the Boltzmann
equation along with three complementary numerical techniques (Molecular
Dynamics, Direct Monte Carlo Simulation Methods and iterative solutions of
integro-differential kinetic equations). For the freely cooling fluid, we
investigate in detail the scaling properties of the bimodal velocity
distribution emerging close to elasticity and calculate the scaling function
associated with the distribution function. In the heated steady state, we find
that, depending on the inelasticity, the distribution function may display two
different stretched exponential tails at large velocities. The inelasticity
dependence of the crossover velocity is determined and it is found that the
extremely high velocity tail may not be observable at ``experimentally
relevant'' inelasticities.Comment: Latex, 14 pages, 12 eps figure
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